This invention relates to a process for preparing halogenated cyclobutenoarenes. In a specific aspect, the invention relates to a simple, low-temperature process for preparing 4-halobenzocyclobutenes in good yields without the side production of large quantities of chemical by-products and waste.
The four-membered ring of benzocyclobutenes is known to open at elevated temperature to form a very reactive diene which rapidly dimerizes and polymerizes. Molecules containing two or more benzocyclobutene groups are therefore useful as heat-curable thermosetting resins. Also, elastomers or thermoplastics containing benzocyclobutene substituents crosslink on heating. Among the more useful intermediates for preparing benzocyclobutene resins or benzocyclobutene-functionalized elastomers or thermoplastics are the 3- or 4-halobenzocyclobutenes, which undergo the many reactions of aromatic halides such as Grignard and Ullmann reactions.
However, the preparation of such halobenzocyclobutene starting materials poses a number of practical problems. Under the usual conditions of halogenation of the benzene ring, opening of the four-membered ring of the benzocyclobutene molecule tends to compete strongly with substitution on the aromatic ring. The ring-opening side reaction is a particularly significant problem under conditions of high acidity and high temperature, as are conventionally used in processes for the halogenation of aromatic hydrocarbons using, for example, a ferric bromide or ferric chloride catalyst.
It is possible, as illustrated in Example 6 of U.S. Pat. No. 4,540,763, to obtain reasonable yields of 4-halobenzocyclobutenes under relatively mild reaction conditions. As described therein, 4-bromobenzocyclobutene can be prepared by the bromination of benzocyclobutene for four days with a large excess of pyridinium bromide perbromide, catalyzed by mercuric acetate. A relatively good yield of isolated product (66%) was obtained. However, the long reaction time, the toxicity of mercury and the large quantity of waste pyridine (and waste HBr, from which the pyridine must be freed by alkali neutralization) are major disadvantages of this procedure.
A somewhat simpler procedure for brominating benzocyclobutene has been published in Tetrahedron, 21, 245-254 (1965). This procedure uses elemental bromine in 95% aqueous acetic acid with an iodine catalyst. A 78% yield was obtained from a synthesis using less than 5 grams of starting material. However, commercial use of such a process would be uneconomical because the amount of acetic acid used is more than ten times the amount of benzocyclobutene starting material used. The acetic acid keeps the product bromobenzocyclobutene, by-product HBr and side products in one phase throughout the reaction. Attempted distillation of the product mixture to remove acetic acid and HBr would require elevated temperatures, which would have the undesirable effect of promoting the opening of the benzocyclobutene ring by the HBr, thus consuming the product. Low-temperature isolation of the product would require extraction of the acetic acid into a basic aqueous solution, which would require large quantities of alkali and would produce large amounts of waste from which recovery of the acetic acid and HBr would be expensive.
It is therefore an object of the invention to provide a process for the preparation of cyclobutenohaloarenes. In one embodiment, it is an object of the invention to prepare halogenated benzocyclobutenes in a low-temperature process which produces relatively small quantities of waste by-products and does not require an acid medium for the halogenation reaction.